1. Field of the Invention
The present invention pertains to polishing of thin flexible workpieces, such as semiconductor wafers, and in particular to carriers used to support the wafers during chemical/mechanical polishing.
2. Description of the Related Art
It is important during chemical/mechanical polishing operations that the slurry have a uniform effect on the semiconductor wafer, during a processing cycle. It has been observed that slurry tends to build up in the region where the wafer peripheral edge, ring extension and carrier plate meet. Elaborate precautions are taken in designing carriers and slurry compositions to ensure the continual flow of slurry about a wafer. This is important, in part, because the abrasive particles within the slurry are "worn down" over prolonged use, and also because of temperature variations (local "hot spots"). It is important that these slurry-related parameters remain uniform throughout all portions of a wafer, during an ongoing polishing operation. Careful inspection of wafer polishing operations indicates that slurry flow is markedly slowed and at times even stagnated in a "build-up" region where the wafer peripheral edge, ring extension and carrier back-up plate meet. In order to better control polishing operations, advances are being sought to remedy slurry stagnation.
Wafer carrier assemblies of the type to which the present invention is directed include a backing plate member having a central protruding portion which provides backing support for the wafer, and an outer surrounding ring mounting portion which is recessed away from the wafer surface. Ring extensions, typically in the form of a band of material having a vertically elongated cross-section, are secured at their upper end to the ring mounting portion of a backing member. The lower free end of the ring extension is aligned with the cross-sectional mid-plane of the wafer or a point slightly therebelow, but yet spaced above the surface of the polishing table.
In order to provide a commercially practical precision alignment of the ring extension with respect to the wafer, the ring extension is undersized so as to have a reduced cross-sectional height, and the amount of undersizing is made up with one or more annular shims positioned between the upper end of the ring extension and the mounting portion of the backing member. In the past, under certain polishing conditions, slurry was observed to migrate into the area of the shims and, on occasion, was found to gain entry between the shims, thus distorting the planarity or planar alignment of the ring extension with respect to the polishing surface. Despite the recognition of this problem, emphasis was still given to the need to provide a free slurry flow across the surface of the wafer to be polished, a concern which oftentimes outweighed the occasional shim distortion problem. Even if the entry of slurry material into the shim area is infrequent, it would be desirable to eliminate the problem altogether, if a way could be developed which would avoid impeding slurry flow across the wafer.
A similar problem has been observed when slurry builds up in the upper portion of the extension ring, between the extension ring and the backing member, at a point above and behind the wafer being polished. On occasion, downforce pressures needed for successful polishing cause slurry to become impacted between the ring extension and the backing member. Upon subsequent ingress of slurry under pressure, slurry was found to build up behind the wafer, that is, between the wafer and the backing member. Again, the problem has been observed on an irregular basis, and in general, it is preferable to assure free slurry flow across the wafer surface being polished. However, a resolution to slurry build-up behind the wafer would be desirable if an adequate solution could be found which avoids interrupting the slurry flow across the wafer being polished.